The present invention relates to an improved process for treating anode slime from copper electrolysis.
In the electrolysis of crude or blister copper for obtaining electrolytic copper, the byproduct anode slime resulting therefrom contains valuable metals such as gold, silver, copper, lead, selenium, and the like. The recovery of these valuable metals by treating such anode slime is of great importance in a copper refinery process.
Methods of treating the anode slime to recover the valuable metals contained therein which have been heretofore proposed comprise extremely complicated processes wherein these valuable metals are extracted one by one and gold and silver are finally recovered, although these methods may be somewhat modified depending upon the composition of the anode slime.
The main factor making the recovery process complicated is, inter alia, the presence of lead which is usually contained in an amount of about 10 to 30 % in the anode slime. For example, in the roasting process for recovering selenium, the presence of lead may reduce volatilization of the selenium. Also, in recovering gold and silver, some of the reduced lead must be re-oxidized and separated in the subsequent cupellation process. Such repeated oxidation and reduction of the lead yields a large amount of semiprocessed products such as fume ash and litharge composed mainly of lead which tend to contribute to complication of the recovery process.
Various methods have been hitherto proposed to simplify these complicated treatment processes and to recover the aforementioned valuable metals more economically and advantageously. These methods include subjecting the anode slime to flotation to remove substantial amounts of the lead contained therein before entering into the actual recovery process. For example, one of these methods comprises oxidizing a suspension of the anode slime in a sulfuric acid solution (about 200g/l) at a temperature above 70.degree.C by passing air therethrough while it is being stirred in order to leach copper therefrom and subjecting the remaining anode slime to a flotation treatment, as is illustrated in FIG. 1.
However, these methods are disadvantageous in that the net yields of gold, silver, selenium and the like carried into the floated material is as low as 85 to 95%, and the separation of lead as tailings, which are their main objects, are unsatisfactory, whereby, they have not yet been put into practice.
Since the anode slime usually is in the form of fine particles having a particle size not greater than 50.mu., the slime has been subjected to a copper leaching treatment or a flotation treatment without having undergone any mechanical treatment such as grinding and crushing in the conventional methods. We have examined sections of anode slime particles under a microscope and found that the anode slime particles are composed of aggregates each made up of finer particles as is shown in the photomicrograph of FIG. 6, and each finer particle is composed of an outer layer comprising silver selenide and gold telluride and a core comprising lead in the form of lead sulfate. In other words, the lead is present as lead sulfate which is wrapped by the outer layer. In the photomicrograph, the white portion represents the silver selenide and gold telluride, while the gray-black portion wrapped by the white portion represents the lead sulfate.
We have made various studies to develop a method to separate the wrapped lead sulfate from the outer layer consisting of the silver selenide and gold telluride, and, as a result, have arrived at a conclusion that frictional and abrasive action by attrition is especially useful for this purpose.